It was overkill but I bought a 9 meter 10mm think stainless cable that you might be able to see on the left pannier and an alarmed padlock.
When camping it was threaded through the rack, rear wheel, chassis and pillion grip and locked with an Oxford alarm boss (?) padlock with a tape cover bike cover. The bike was also parked next to the tent (out of falling range). The panniers were in the tent with us at night. If we left the bike at a camp-site while using public transport we would lock the panniers on the bike, thread the cable through the pannier handles and through the bike as above.
When parked in a city or some where we could not see it we would thread the cable through everything as above but also to something like a telephone poll or other serious object. We occasionally left our jackets with the helmets on the bike which was also threaded with the cable.
We also had a packsafe 55 (stainless mesh with a drawstring type wire) which held the tent on top of the topbox. That's the blue thing on the top box in the photo.
The tank bag had a valuable electronics and paper work. That came with us everywhere.
As others have said all this would not stop a determined thief but prob would make them think twice and prob move on to something better than a dirty looking 1995 Yamaha diversion..The photo is at the start of that trip so still pretty clean. Insurance or not the bike going missing would ruin a trip.